Twin blade mounting for fans and propellers



E. M. TURNER TWIN BLADE MOUNTING FOR FANS AND PROPELLERS Filed May 28,1948 Feb. 26, 1952 INVENTOJE:

Z11.Turnr OF P 747/0 d wm Patented Feb. 26, 1952 UNITED STATES PATNTorrlcs v Estey Turner, Arcadia, Calif.

Application May 28, 1948, Serial N0. 29,816

6 Claims.

This invention relates to variable pitch air impellers and hasparticular reference to improvements in the mounting of the blades ofsuch impellers.

In airplane propellers and circulating fans, single blades areordinarily used and it has become common practice in order to providemore efficient air flow control for particular purposes to mount theblades for axial rotation on the impeller hub thereby to vary the pitchof the impeller during operation thereof. In performing the operation ofrotating the blades to vary the pitch in the single blade impellers, itis found that certain forces, commonly known as the centrifugal torque,exert pressure against such single fan blades to, urge axial rotation ofthe blades into neutral position. It is furthermore found that thepressure in this manner exerted increases as the speed of rotation ofthe impeller in creases.

It is largely for the purpose of overcoming or balancing these forcesthat twin blades have been introduced for use in air impellers and it isthe object of the present invention to devise the most efficient mannerof mounting and relatively arranging the units of such twin blades.

In mounting the units of twin blades in parallel spaced relation to eachother and having their leading edges rectangularly aligned, it has beenfound that while theoretically the twin blade is more efilcient than thesingle blade, this advantage is largely sacrificed by the relativepositions of the units, the reason being that the front unit of theblade blankets the rear unit to such an extent that it substantiallydestroys the utility thereof. It is in view of this condition thefurther object of the invention to arrange the units of the blade insuch staggered relation to each other that the front unit of the bladeis set back a considerable distance relative to the rear unit. When so.arranged, it is found that the front unit acts as a booster for the rearunit, resulting in an increase in efficiency as high as 40% at certainblade pitches, as has been determined by numerous wind tunnel tests andby a series of small propeller tests.

Further study and a great number of tests have taught me that therelative angle of incidence of the blade units is an important factor inthe efficiency of the impeller and it is an additional object of theinvention so to arrange the angle of incidence of the units that thegreatest efficiency of each particular blade assembly is obtained.

These and other objects of the invention are hereinafter fully describedand drawings are hereto annexed in which a preferred form of theinvention is illustrated.

In the drawings:

Fig. 1 is a front view, partly in section, of a twin blade combinationembodying the invention as it appears when the blade is turned intoneutral position;

Fig. 2 is a plan view of the lower portion of the blade and bladesupporting members illustrated in Fig. 1, as they appear when removedfrom the impeller hub thereof.

Fig. 3 is a schematic view which is added more clearly to illustrate therelative position of the blade units to each other.

As indicated in Fig. l, a twin blade composed of units 1, 2 is mountedon a propeller hub 3. A stem 3 projects radially from the hub and theouter end of this stem is threaded to receive a nut 5 by means of whichthe blade units are clamped in position on the stem. As usual inpractice, this stem is rotatable by suitable, wellknown external meansto vary the pitch angle of the twin blade assembly to suitoperating'condi tions. And it should be understood that there is one ofthese stems for each propeller blade. The units of the blade are madefrom thin sheet metal, the inner ends of which are shown rigidly securedto brackets 6, 1, which are centrally perforated to seat'on the stem 4.As best shown in Figs. 2 and 3, each blade unit is slightly transverselycurved, such curving having been found more efficient than theconventional flat blade. It is important to note that the portion ofthese brackets to which the blade units are secured do not extend theentire distance across the blade unit, but that the center portionthereof is removed, leaving legs from the ends of which lugs 6 5, and ll rise perpendicularly. These lu s are merely wide enough to retainsuflicient strength to support the blade units. The purpose of soshaping the brackets is to obtain as nearly as possible a perfectlybalanced condition. If the portion between the lugs at the ends of thebrackets had not been removed, it would be necessary, in order to obtaina balanced condition, to place similar panels between the lugs ofadjacent legs of the two brackets, as indicated at 8 and 9 in dottedoutline in Fig. 2. It may be suggested that the blade units I, 2 show asimilar unbalanced condition and there is no doubt but that, if thecentral portion of the blade units were similarly removed, a moreperfectly balanced assembly would be obtained, but it has not yet beenfound commercialy practical to produce such four unit blade and, in thepresent state of development, it becomes necessary to stay within thelimits prescribed by accepted manufacturing methods.

As suggested above, Fig. 3 is added for the purpose of more clearlyshowing the relation of the blade units to each other, to the directionof the air flow and to the direction of rotation of the impeller.Experience and numerous tests have proved to me that the distancebetween the units of the twin blade holds a certain fixed relation tothe width of the blade units and it has been my experience that mostsatisfactory results have been obtained where the width of the units isabout twice the distance between the units. As above stated, it isnecessary to set the leading edge of the front unit 2 back relative tothe direction of propeller rotation a considerable distance behind theleading edge of the rear unit I in order to permit the leading edge ofthe rear unit first to act upon the air and to impart motion thereto,concurrently to create a partial vacuum on the back surface of thisblade unit. The distance that the leading edge of the front unit is setback should be at least equal to one fourth of the distance between theunits.

Referring further to Fig. 3, it is noticed that the pitch angle, whichis the mean'angle of incidence of the twin blade relative to thedirection -of rotation thereof corresponds to the various positionstaken by the blade during operating conditions and it is important tonote that the angle of incidence of the front unit 2 of the twin bladeis greater than that of the rear unit I. I have found that thisincreased angle of incidence of the front unit has a controlling effecton the vacuum formed on the rear unit of the twin blade and also impartsincreasing velocity to the air stream which has been set in motion bythe leading edge of the rear blade unit. In other words, I have foundthat the front unit of the blade, on account of its increased angle ofincidence, acts as a booster resulting in an increase in efficiency ashigh as 40% at certain pitch angles of the blade. This result, asstated, has been definitely determined by numerous wind tunnel tests andby a series of other impeller tests conducted by me over a long periodof time. Once the correct angle of incidence has been fixed, the twosupports may be positively locked in position against relative rotationby means of a screw or pin I0.

I am aware that staggering of blade units in a twin blade propeller hasbeen proposed but, in such devices as have come to my notice, I havefound that the front unit of the blade has been given an advance staggerand the rear unit has been set back relative to the front unit. Sucharrangement has been found completely impractical and has resulted in adefinite loss of efficiency, as has been conclusively indicated in windtunnel tests. The relative positions of the blade units of the presentdevice is therefore of utmost importance.

It was above stated that the distance between the units of the twinblade should be approximately one-half the width of the blade units.Such relative proportioning has also been found most efficient inreducing the effect of the centrifugal torque on the blade duringoperation thereof and to result in easier adjustment of the blades tochange the pitch angle during operation thereof.

I claim:

l. The combination with an impeller hub having a stem radiallyprojecting therefrom, of a twin blade support comprising two brackets,each bracket including a bifurcated base perforated at the apex thereoffor engagement with said stem, the base having legs extending from thebifurcation thereof andhaving lugs perpendicularly rising from the endsof the legs for supporting one of the units of the blade, means forclamping said brackets in position on the stem, and means for clampingthe twin blade units in position on the lugs of the brackets.

2. The combination with an impeller hub having a stem radiallyprojecting therefrom, of a twin blade support comprising two brackets,each bracket including a bifurcated base perforated at the apex thereoffor engagement with said stem, the bracket having legs extending fromthe bifurcation thereof and having lugs perpendicularly rising from theends of the legs for supporting one of the units of the blade, means forclamping said brackets in position on the stem, and means for clampingthe twin blade units in position on the lugs of the brackets, the lengthof the legs of each bracket and the position of the lugs of the bracketsrelative to the bracket centers being proportioned to space one unit apredetermined distance away from the other unit.

3. The combination with an impeller hub hav ing a stern radiallyprojecting therefrom, 'of a twin blade support comprising two brackets,each bracket including a bifurcated base perforated at the apex thereoffor engagement with said stem, the bracket having legs extending fromthe bifurcation thereof and having lugs perpendicularly rising from theends of the legs for supporting one of the units of the blade, means forclamping said brackets in position on the stem, and means for clampingthe twin blade units in position on the lugs of the brackets, thebrackets being rotatable on the stem to bring the units into correctangular relation to each other.

4. The combination with an impeller hub having a stem radiallyprojecting therefrom, of a twin blade support comprising two brackets,each bracket including a bifurcated base perforated at the apex thereoffor engagement with said stem, the brackets having legs extending fromthe bifurcation thereof and having lugs perpendicularly rising from theends of the legs for supporting one of the units of the blade, means forclamping said brackets in position on the stern, and means for clampingthe twin blade units in position on the lugs of the brackets, the legsof each bracket, the

positions of the lugs of the brackets relative to the bracket centersbeing proportioned to spaceone unit a predetermined distance away fromthe other unit, the brackets being rotatable on the stem to bring theunits into correct angularrelation to each other, and means for lockingthe brackets against relative rotation. v

v 5. The combination with an impeller hub having a stem radiallyextending therefrom and curved twin blade units mountable on said stem,of a support comprising two brackets, each bracket including abifurcated base perforated at the apex thereof for engagement with saidstem, the bracket having legs extending from the bifurcation thereof andhaving lugs perpendicularly rising from the ends of the legs forsupporting one of the blade units, the lugs being curved to fit thecurvature of the unit they support, means for clamping the units inposition on the lugs, and means for clamping the brackets in position onsaid stem.

6. The combination with an impeller hub having a stem radially extendingtherefrom and curved twin blade units mountable on said stem, of asupport comprising two brackets, each rising from the ends of the legsfor supporting 5 one of the blade units, the lugs being curved to fitthe curvature of the unit they support, means for clamping the units inposition on the lugs, the length of the legs of each bracket and theposition of the lugs of the brackets relative to w the bracket centersbein proportioned to space one unit a predetermined distance away fromthe other unit, means for clamping the brackets in position on saidstem, the brackets being rotatable on the stem to bring the units intocorrect 15 angular relation to each other, and means for locking thecorrectly set brackets againstrelative rotation.

ESTEY M. TURNER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 10,474 Leach Jan. 31, 18541,021,822 Broussouse Apr. 2, 1912 1,098,306 Trautmann May 26, 19141,243,885 Smyth Oct. 23, 1917 1,355,462 Francis Oct. 12, 1920 1,463,441Pike July 31, 1923 1,813,877 Gunn July 7, 1931 1,864,492 KrasnodmskyJune 21, 1932 2,279,190 Wilkinson Apr. 7, 1942 2,344,266 Reissner Mar.14, 1944

